Metallic flame radiation



Jan. 10, 1956 c. KENTY 2,730,645

METALLIC FLAME RADIATION Filed Sept. 3, 1953 lnventor; Carl Kent HisAttorney METALLIC FLAME RADIATION Carl Kenty, Cleveland Heights, Ohio,assignor to General Electric Company, a corporation of New YorkApplication September 3, 1953, Serial No. 378,242

7 Claims. (Cl. 313-135) This invention relates in general to theproduction of radiation by the excitation of metallic spectra. Moreparticularly, it relates to the production of light by the use of activenitrogen to excite metal vapors to spectral emission.

An object of my invention is to provide a novel light source.

7 Another object of my invention is to provide a new and improved meansfor exciting metallic spectra.

In accordance with my invention, I have discovered that under certainconditions the metallic spectra of the electrodes of discharge devicesmay be excited to produce flames of appreciable luminosity. Theexcitation occurs when short high current discharges are caused to flowbetween the electrodes in an envelope containing a gaseous fillingcomprising an inert gas at a pressure in excess of one centimeter ofmercury and a small percentage of nitrogen. The electrodes areunactivated and preferably maintained cold to assure copious sputteringof the electrode metal. Thus, when a discharge occurs, the electrodesare surrounded with metal vapor whereof the atoms are excited tospectral emission by the active nitrogen simultaneously produced by thedischarge.

Further objects and advantages of my invention will appear from thefollowing description taken in conjunction with the accompanyingdrawing. The features of my invention believed to be novel will be moreparticularly pointed out in the appended claims.

In the drawing:

Fig. 1 is a schematic illustration of an electric discharge device andoperating circuit therefor for the production' of metallic spectra inaccordance with my invention.

Pig. 2 is a longitudinal sectional view through an electric dischargedevice for the production of metallic flame spectra and embodying myinvention.

The metallic spectra or flames produced in accordance with my inventionappear to be excited by the active nitrogen produced by the discharge.Active nitrogen as such has been known for many years. An admirableaccount of its physical. and chemical properties, and its history, isgiven in Active Nitrogen-A New Theory by S. K. Mitra, 1945, published bythe Indian Association for the Cultivation of Science, Calcutta, India.If a condensed spark, that is an oscillatory discharge, is passedthrough nitrogen at a pressure of the order of one-millimeter, ayellowafterglow will be observed. This yellow glow consists of aselection of the so-called first positive bands of nitrogen.

Many, theories have been advanced to explain active nitrogen, an accountof some of which is given by S. K. Mitra above. According to, one theoryfirst suggested by A. G. Gaydon and further elaborated by R. W. Nichollsin the Journal of Chemistry and Physics, 20, 1040 (1952), activenitrogen is a mixture of three and possibly four kinds of metastablenitrogen (N2) molecules. These molecules have a long life. One of themwith the aid of a collision of another body, a free elecnited StatesPatent 9 l 2,730,645 Patented Jan. 10, 1956 tron according to Nicholls,is converted into another kind of excited state of nearly the sameenergy, which radiates the familiar yellow and red bands of activenitrogen. This reaction takes place very slowly and so the afterglow hasa long life.

I believe that some or all of these metastable nitrogen molecules causethe excitation of the metal atoms to produce the flames above mentioned.However, in order to produce flames of appreciable intensity andluminosity, certain conditions must be observed. Referring to Fig. 1, asuitable apparatus for achieving the conditions in accordance with myinvention is diagrammatically illustrated. The discharge devicecomprises a vitreous envelope 1 into which is sealed a pair ofelectrodes 2, 3 in the form of metal spheres mounted on the ends of leadwires 4, 5 and defining a short are gap. The spherical electrodes 2, 3are unactivated and will consist of the metal whereof the spectralemission is desired, for instance tungsten productive of a blue flame.

To produce a good flame, that is one of high luminosity, the gaspressure and composition must be properly se lected in accordance withmy invention and also the electric discharge must have the propercharacteristics. Since the flame is produced by the excitation of thesputtered metal from the electrodes, it is desirable to have copioussputtering. I have found that a favorable gas mixture for the flamesmust comprise in excess of 3 centimeters of mercury of an inert gas witha small percentage of nitrogen. For instance, a suitable gas mixture mayconsist of argon at a pressure of 10 to 50 centimeters of mercury, with1 to 3 per cent of nitrogen added. These quantities are not critical,but at a pressure below 10 centimers of mercury the degree of electrodesputtering decreases, and below 3 centimeters the amount of sputteringis too low to produce a flame of appreciable luminosity.

The spacing of the metal electrodes 2, 3 is not critical. However, sincethe sputtering of the electrode metal is the effect sought, it will beappreciated that a long arc gap is unnecessary and merely increases thevoltage required to produce the discharge. I have found that an arc gapless than 1 centimeter in length, for instance a spacing of 1 to 4millimeters between the spherical electrodes 2 and 3, is suitable forthe purpose of the invention.

For high luminosity of the metallic spectra, the dis charge must consistof short pulses of current of high intensity. It has not as yet beendetermined whether a discharge of this nature is desirable because itproduces a greater degree of electrode sputtering or because it is moreeflicient in producing active nitrogen. Probably it is desirable onaccount of both reasons. In any case, my experiments have clearlydemonstrated that the quantity of light produced when a given quantityof electrical energy is discharged through the tube in the form of shorthigh intensity pulses, is many times greater than when the same quantityof energy is discharged through the tube as a uniform low intensitycurrent.

The most favorable type of discharge which I have found is a condensedspark or damped oscillatory discharge. Rcferring to Fig. 1, such adischarge is produced by means of a high voltage transformer 6comprising a primary winding 7 connected to a variable autotransformer8, which is in turn energized from the usual to volt, 60-cyclecommercial alternating current supply. The secondary winding 9 oftransformer 6 may provide a maximum open circuit voltage of 10,000 voltsand a short circuit current of 30 milliamperes. There is a common neonsign transformer which meets the above stated requirements fortransformer 6. The secondary winding 9 of the transformer is connectedin series with an external air gap 11 across electrodes 2, If; ofdischarge device 1." 'AbapacitorIZ which may have a rating ofapproximately 500 micromicrofarads is shunted across ssqa ary Win in hexternal a p. .1. nroterablv hou have e ectr des o s lve o tung ten to. pvent. x ssive e n a ayave. fo nd that f a giv n i ap. e th e r nsformervoltage should be a justed (by sliding the autotrans former tap) to aslow a value as possible while yet avoiding failure to. spark. A sort ofhissing noise emanates from air gap 11 under these circumstances.

The most copious sputtering and the most luminous flames are obtainedwhen the electrodes are cold. Since con i u S a i w l s heat ng f the er ome me t r dne t s adv tag ous t use th snark intermittently. It willbe appreciated, of course, that i m e. do e by usin a. r ta ing switchor other equivalent device, if so desired. Furthermore, the heatdissipating capacity of. the electrodes may be increased by any one ofthe methods well-known, in the. art, for instance by use of heayierlead-in wires and radiating fins.

In order to obtain the pure spectrum of the electrode metal, it isnecessary to have a very good exhaust of the bulb previous to thefilling with the argon and nitrogen mixture. Otherwise, it will befound. that the flames are colored violet by the cyanogen (CN), bands.Preferably, the exhaust should include a high, temperature. baking, forinstance to 450 C. for one hour. Then. the electrodes should be heatedto bright redness in oxygen at 5 to millimeters pressure in order toremove entirely all carbon and carbonaceous gases yielding cyanogen.Glow discharge heating is satisfactory for this purpose. Further heatingof the electrodes in hydrogen at 5. to 10 millimeters pressure reducesthe oxide and a final heating in argon drives out absorbed hydrogen.

When the switch 10 is, closed to energize transformer 6, the dischargeoccurring through the device 1 is of the type generally known asacondensed spark discharge. A series of sparks occurs across the airgap. 11, and each spark is in the nature of a. dampedhighfrequencyoscillation. With the circuit constants that. have been stated,the oscillatory frequency is inthe range of. lOmegacycles per second.The discharge is highly damped, and thus the current flow through thedevice 1 is in the nature of a continuous seriesrof short highintensitypulses. it will be appreciated that a discharge of this character causesconsiderable radio and. television interference. To minimizesuchinterference, the tube 1 shouldpreferably. be operated inside a groundedmetal screen enclosure, and the air gap 11 along with the circuitapparatus. should also preferably be soenclosed.

The flames which areproduced by-the discharge have been indicated withinthe tube 1 insofar as drawing limitations will permit. In thearc gapspace immediately between the spherical electrodes 2, 3, there occurs abright spark 14 of high intensity which shows the arc spectrum lines ofargon and nitrogen and also the spectral lines of the electrode metal.Surrounding the electrodes, there is a bright luminous cloud 15 whichlooks remarkably like a flame and which containsthe spectral lines ofthe electrode metal. Surrounding the bright cloud or flame 15, there isluminous region ofmuch lesser intensity in dicated at 16 which shows theyellow and red bands of active nitrogen.

The color and appearance of the bright cloud or flame 15 will dependupon the metal selected for the electrodes 2, 3. I have produced theflames of many different metals. Some of the outstanding ones are:tungsten, giving a blue flame; vanadium, red; columbium, yellow;palladium, green; chromium, blue-violet, and iron, violet.

Of the various metals which I have tried, columbium gives the highestluminosity. I have estimated the light producing efliciency of such adevice with columbium electrodes to be of the order of one lumen. perwatt. For the production of spectacular lighting effects or for the rrodot or metal i s ectra. h present di ch device is a highly effectivesource. The flame spectrum consists of the line spectra of the metalatoms of the electrodes. However, the spectral lines in the flames havea somewhat different intensity distribution as compared to thoseobtained in an ordinary are between the given metal electrodes in air.

Referring to Fig. 2, there is illustrated a discharge device embodyingthe invention wherein a plurality of electrode gaps are mounted inseries within a single envelope. The device comprises a tubular vitreousenvelope 21 having stems 22 and 23 at opposite ends through which aresealed a pair of terminal electrodes 24, 25. in between the ends of theelectrodes. 24 and 25 are supported a plurality of spaced intermediateelectrodes 26 which, together with the end electrodes 24, 25, form aseries of arc gaps 27 distributed across the length of the tube. Theintermediate electrodes 26 are supported in place by short support wires28 which are preferably spot-welded to the electrodes near theirmidpoint; the rower ends of the support wires are embedded; in a glasssupport rod 2). The rod 29 may be held in place within the tube by meansof auxiliary support wires 31 embedded into tie stem presses 22, 23 atthe ends of the tube 21.

When, the discharge device of Fig. 2 is energized in an electricalcircuit similar to that illustrated in. Fig. l, a bright flame of theelectrode metal spectrum forms about each arc gap 27. If the endelectrodes 24-, 25 and also the intermediate electrodes 26 are all ofthe same metal, the flames about each. gap will all be of the samenature. If desired, the portions of the electrodes adjacent the variousgaps may be made of difierent metals and then a number of vari-coloredflames may be obtained within a single envelope and the results. arevery pretty.

Whereas in thespecific embodiments of the invention which I have.described, I have stated the inert gas to be argon, the. other inertgases may likewise be used. Helium and neon give flames. about as brightas that obtained with argon and. of identical spectral quality. Kryptongives a flames fully as bright, but of different spectralquality. Xenon,however, gives a relatively weak flameof considerably different spectralquality.

When xenon is used as a rare gas, I have observed a rather surprisingresult which is not present with any of the other rare gases. Withxenon, the yellow afterglow of active nitrogen is not obtained under anycircumstances. This would appear to indicate that the metastablenitrogen. state, mentioned earlier as the exciter of the yellow activenitrogen. bands, is losing its energy to the xenon atoms and excitingthem instead of the nitrogen. In one experiment wherein I used a gaseousfilling consisting of argon at 20 centimeters of mercury, xenon at 3centimeters, and nitrogen at 1 centimeter, there was very little yellow.afterglow of active nitrogen, but a mild after-fluorescence of theglass. Such a fluorescence of the glass is not observed in any of theother cases, and I believe that it is due to the resonance radiation ofxenon as excited by the nitrogen metastable molecule in question.

The above-described property of xenon may be made use of-to eliminate,when desired, the yellow glow and afterglow which. surrounds themetallic spectrum flame in the devices of Figs. 1 and 2. The yellow glowsometimes detracts from. the beauty. of the flame, and by adding somexenonto the argon. and nitrogen mixture, the yellow glow can be entirelysuppressed without apparently diminishing the intensity of the flameitself. A suitable gas filling for this purpose may consist of argon at20 centimeters of mercury, xenon at 3 centimeters, and nitrogen at 1centimeter.

While certain. specific embodiment of the invention have been shown anddescribed, various modifications and other applicationsof; the inventionwill readily so our to those skilled in theart. The appended claims are.therefore intended to cover any such modifications coming within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of terminal electrodes of unactivated metal, aplurality of intermediate unactivated metal electrodes insulativelymounted within said envelope and spaced end-to-end between said terminalelectrodes and with short are gaps between adjoining electrodes, and agaseous filling within said en velope comprising an inert gas at apressure in excess of 3 centimeters of mercury and a small percentage ofni-- trogen.

2. An electric discharge device comprising an envelope having sealedtherein a pair of unactivated metal electrodes defining an arc gap, anda gaseous filling Within said envelope comprising an inert gas at apressure in excess of 3 centimeters of mercury and a small percentage ofnitrogen.

3. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of unactivated metal electrodes defining an arcgap, and a gaseous filling Within said envelope comprising an inert gasat a pressure in the range of to 50 centimeters of mercury and including1 to 3 per cent of nitrogen.

4. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of unactivated metal electrodes defining an arcgap, and a gaseous filling within said envelope comprising argon at apressure in the range of 10 to 50 centimeters of mercury and including 1to 3 per cent of nitrogen.

5. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of unactivated metal electrodes defining an arcgap, and a gaseous filling within said envelope comprising argon at 20centimeters pressure, Xenon at 3 centimeters pressure and nitrogen at 1centimeter pressure.

6. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of terminal electrodes of unactivated metal, aplurality of intermediate unactivated metal electrodes insulativelymounted within said envelope and spaced end-to-end between said terminalelectrodes and with short are gaps between adjoining electrodes, and agaseous filling within said envelope comprising an inert gas at apressure in the range of 10 to centimeters of mercury and including 1 to3 per cent of nitrogen.

7. An electric discharge device comprising a vitreous envelope havingsealed therein a pair of terminal electrodes of unactivated metal, aplurality of intermediate unactivated metal electrodes insuiativelymounted within said envelope and spaced end-to-end between said terminalelectrodes and with short are gaps between adjoining electrodes, and agaseous filling within said envelope comprising argon at 20 centimeterspressure, xenon at 3 centimeters pressure and nitrogen at 1 centimeterpressure.

References Cited in the file of this patent UNITED STATES PATENTS790,975 Ovington May 30, 1905 1,350,170 Murphy Aug. 17, 1920 1,799,993Staege Aug. 7, 1931 2,018,347 De Beaufort et al Oct. 22, 1935 2,023,436Prinz Dec. 10, 1935 2,086,668 Fodor July 13, 1937 2,135,718 Krefft Nov.8, 1938 2,154,317 Suits Apr. 17, 1939 2,209,523 Kaiser July 30, 19402,485,037 Clark Oct. 18, 1949

1. AN ELECTRIC DISCHARGE DEVICE COMPRISING A VITREOUS ENVELOPE HAVINGSEALED THEREIN A PAIR OF TERMINAL ELECTRODES OF UNACITVATED METAL, APLURALITY OF INTERMEDIATE UNACTIVATED METAL ELECTRODES INSULATIVELYMOUNTED WITHIN SAID ENVELOPE AND SPACED END-TO-END BETWEEN ADMINALELECTRODES AND WITH SHORT ARC GAPS BETWEEN ADJOINING ELECTRODES, AND AGASEOUS FILLING WITHIN SAID ENVELOPE COMPRISING AN INERT GAS AT APRSSURE IN EXCESS OF 3 CENTIMETERS OF MERCURY AND A SMALL PERCENTAGE OFNITROGEN.